android-system-image: append BUILDDATE to filenames

* symbols.tar.bz2 filename was conflicting between grouper and tuna in downloads dir
* add whole cm version to symbols tarball
* append BUILDDATE also to cm .zip, we want different filename when phablet builds create
  different zip even when it is the same PV

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>